Handover, especially hard handover, may interrupt a user's active communication for a specific time duration known as an interruption time, at the rate of tens and hundreds of ms for intra- and inter-frequency handover (HO), respectively. For some time-sensitive services, the interruption time could deteriorate the service level. Further, HO may cause user data to be lost. For these reasons, the target and serving base stations (BS) should buffer user data and on some occasions coordinate the user data transmission to avoid duplication. More seriously, during HO, the mobile station (MS) may drop off due to a lack of radio resources in the target BS or failure of network re-entry to the target BS, etc. From this perspective, HO should be avoided to some extent, whilst keeping other system performance parameters at reasonable levels.
Inter-cell interference is well known to cause degradation in the signal quality of the MS, especially in the cell edge area. Normally, the HO trigger metric is a Receive Signal Strength Indicator (RSSI) or a Carrier-to-Interference-and-Noise Ratio (CINR). HO usually takes place in the cell edge area, where the majority of interference is caused by neighbouring cells. Therefore, mitigating inter-cell interference may lead to potentially delaying HO and/or decreasing HO frequency.
Inter-cell interference avoidance mitigation (ICIM) can be classified as follows:                Interference avoidance/reduction at the transmitter(s), for example Fractional Frequency Reuse (FFR) and interference randomisation.        Interference cancellation/reduction at the receiver(s).        
FIG. 1 is a schematic diagram showing a wireless network including seven base stations BS0 to BS6. Around BS0 there are shown three zones A, B and C. Zone A is a non-handover area in which a mobile station located in the zone is not to be handed over to any of the other base stations. Zone B is an interference mitigation area in which inter-cell interference mitigation methods are to be performed. Zone C is a hard/soft handover area in which a mobile station is to be handed over to one of the other base stations BS1 to BS6.
As demonstrated in FIG. 1, without ICIM, the HO area may include areas B and C. By adopting ICIM techniques, the HO area may decrease to area C only, in which case HO frequency would be reduced, thus leading to an enhanced cell edge user performance.
As an amendment system for the IEEE802.16e system, the IEEE802.16m system is required to enhance cell-edge user performance, thereby improving system performance. To introduce ICIM, the 16m system should adopt new physical layer mechanisms and manage this scheme through PHY/MAC layer signalling/messages.